What about 02 Wireless which is an MVNO on the ATT network and calls to UK landlines are charged at the domestic rate. Here is a link to an EBay seller of them:

http://cgi.ebay.com/Oxygen-O2-Wirele...#ht_7206wt_988

Tmobile has a prepaid at 10 cents per minute and free calls to US landlines from 7pm to 7am. If you combine it with a cheap calling card, that is great deal if you are willing to make calls to the UK in the early morning.

Now, sign up for a free UK landline number from sipgate.co.uk and buy some minutes. It is cheap to call forward that number to the US, or buy a UK virtual number from Skype and their $2.95 per month unlimited US calling and divert that number to your US mobile when you are not available.